1. Field of the Invention
This invention relates generally to a process for preparing aqueous compositions including inorganic material particles, and more particularly to the dispersion of inorganic material particles in products formed by the loss of water from such aqueous compositions.
2. Background of the Invention
Aqueous dispersions of polymeric latex are used to prepare a tremendous variety of commercially important products, including coatings, primers, mastics, caulks, binders for non-woven materials, and adhesives. Frequently, the polymeric latex is included to form a continuous phase as the water is lost in order to bind the product together, and to contribute important physical properties. In addition to the polymeric latex, most products include particles of one or more inorganic materials. Some inorganic materials contribute an important functional property to the product, such as fire resistance, sheen, flatting, hardness, color, texture, or toughness. Often these inorganic materials are relatively expensive, and it is desirable to employ them in the formulated product as efficiently as posssible. In other instances, the inorganic materials, for example, fillers, may be less expensive than the latex and it may be desired to incorporate the maximum amounts possible without compromising the desirable properties attributable to the latex, such as, for example, binding and tensile properties, such as elongation and, in the case of adhesives, minimum loss of tackiness. Since the polymeric latex is typically the most expensive component of the formulated product, inexpensive inorganic materials are frequently included to reduce product cost. In this case, it is desirable to use the greatest possible volume of the inexpensive inorganic material, while still retaining the desired performance properties of the product, since these properties typically decline when too much of the inexpensive inorganic material is included. A good example is a coating formulated with so much of an inexpensive filler pigment, such as calcium carbonate, that the critical pigment volume concentration is exceeded. Such a coating will provide a much less protective coating for a surface on which it is applied than a similar coating formulated with slightly less filler, and having a pigment volume concentration below the critical level.
One way of making efficient use of the inorganic material employed is by dispersing this pigment as well as possible. Agglomerated or aggregated inorganic material pigment particles frequently make less than their full potential contribution to the performance of the product, such as, for example, strength and resistance properties. Inorganic materials are typically sold as dry powders. As a practical matter, this powder must be milled in a liquid medium to break up agglomerates and to obtain a colloidally stable dispersion.
In order to stabilize the product formulation against settling or flocculation, a variety of surface active dispersing additives have been used. Manufacturers have often used inorganic materials as dry powders. In this case, a dispersing additive is usually added directly to the material and a small amount of water in a preliminary "grind" step in which loose agglomerates of the inorganic material particles are broken up by mechanical shear forces. The dispersing additive typically remains in the mix through the subsequent steps in the product preparation process and typically will be found in the fully formulated product composition.
Sometimes inorganic materials are commercially supplied in concentrated slurry form, such as, for example, in water. However, since the inorganic material particles in the slurries are prone to aggregation upon storage, the slurries often need to be redispersed for maximum effectiveness before use in formulating a coating composition. Either the manufacturer of the inorganic material slurry or the end user of the slurry, and sometimes both, may perform the redispersion step. This step is another source of dispersing agent in the fully formulated product.
The traditional guiding rule or goal in the formulation of practical, dispersed inorganic material-containing products, such as coatings, is to make the inorganic material dispersion and the polymeric latex binder dispersions as colloidally stable and compatible with each other as possible, in the sense that they can be mixed without formation of coagulum or like aggregate or excessive increases in viscosity. It has heretofore been found that inorganic material particles cannot be effectively dispersed into aqueous latex compositions by simply blending or directly mixing them into the aqueous polymeric latex composition. When such direct blending of inorganic material particles has been attempted, agglomeration of the particles has resulted in the formation of grit or coagulum in the composition. Products containing grit or coagulum may suffer the loss of properties such as gloss, mechanical strength and impermeability. Even in the case of nonaqueous (solvent-borne) or 100% solids polymer compositions, high energy grinding or milling input has been found to be necessary to deagglomerate and wet out the inorganic material particles. Even with the use of such operations, inferior dispersions containing aggregates of inorganic material particles result unless significant quantities of dispersing, wetting or "coupling" agents are employed.
In the case of many compositions including polymeric latex, such as polymeric latex adhesives, and latex coatings, it is conventional practice to first form a stable aqueous dispersion of the inorganic materials including the pigments, fillers and extenders. This dispersion, also known as a "grind" or "millbase," may contain water-miscible solvents, such as for example glycols and glycol ethers, and relatively low molecular weight water soluble polyelectrolytes as inorganic material pigment grinding aids or dispersants. Generally, these pigment dispersants are anionic polyelectrolytes. Many different types of such dispersants are known. For example, U.S. Pat. No. 2,930,775 discloses the water soluble salts of diisobutylene maleic anhydride copolymers having molecular weights between about 750 and 5,000 as dispersants when employed at concentrations of from about 0.05 to 4% on pigment weight. U.S. Pat. No. 4,102,843 and U.S. Pat. No. Re. 31,936 disclose the use of water soluble salts of copolymers of hydroxyalkyl (meth)acrylates and (meth)acrylic acid of molecular weights of from 500 to 15,000 at concentrations of from about 0.01 to 5% on pigment to produce glossy emulsion coatings. U.S. Pat. No. 4,243,430 discloses a water-soluble dispersant comprising an addition copolymer comprising greater than 30% alpha, beta-unsaturated monovinylidene carboxylic acid, the copolymer having an apparent pK.sub.a between 6.0 and 7.5 and molecular weight between about 500 and 15,000, and forming a water soluble salt with zinc ammonia complex ion. Low molecular weight polyphosphate salts, such as potassium tripolyphosphate, are also used because they are relatively inexpensive, but they tend to have marginal hydrolyric stability.
The use of these and other polyelectrolyte dispersants is described in T. C. Patton, Paint Flow and Pigment Dispersion (Wiley Interscience, 2nd edition) 290-295 (1979). Also described therein (pages 468-497) are a number of milling devices used in the preparation of pigment dispersions. One such device which is commonly used in the manufacture of latex paints is the high speed disk disperser designed to develop high shearing forces in the pigment grinding step. Common practice is to use the device with dispersant to form a stable dispersion of inorganic material pigment, and then to add to the dispersion the aqueous latex polymer binder along with the other ingredients, such as for example thickeners and rheology modifiers, antifoaming agents, colorants, coalescing agents or temporary plasticizers for the latex polymer particles, and surfactants for substrate wetting and colorant comparability. The pigment dispersion process and the relation to flocculation and other properties are discussed further in Treatise on Coatings, Vol. III, Part 1 (Marcel Decker) (1975); Pigment Handbook, Vol. I (2nd edition, Wiley Interscience) (1988), and Pigment Handbook, Vol. III (Wiley Interscience) (1973). Other types of pigment dispersion equipment are also employed, including ball mills, sand mills, Banbury mixers, and the like, depending on the nature and physical characteristics of the product.
While these polymeric dispersants and procedures enable the formulation of practical aqueous formulations, they have certain undesirable characteristics. Because of their ionic nature, polyelectrolyte dispersants tend to impart water sensitivity to films, which can result in reduced resistance of the films to scrubbing, and increased swelling with a tendency,towards delamination and blistering. Similarly, the presence of polyelectrolyte dispersants can reduce the corrosion resistance of coatings films applied to steel surfaces. Moreover, such polyelectrolyte stabilized dispersions of inorganic material particles are prone to flocculation and reaggregation by other ionic species in the aqueous formulation, such as, for example, from initiator residues and from soluble inorganic pigments, especially those which contribute multivalent ions such as zinc oxide and calcium carbonate. Furthermore, since films dry by water evaporation, the concentration of such ionic species in the coating or film increases upon drying, and an otherwise suitable dispersion may become aggregated during the drying process itself. The stability and instability of inorganic material dispersions in the presence of polyelectrolytes and multivalent metal ions has been described by Burlamacchi, et al., Colloids and Surfaces 7, 165 (1982).
A number of techniques have been proposed to disperse inorganic particles such as inorganic material particles in aqueous polymer containing coating compositions.
There is a continuing need to improve the effective utilization of inorganic material in aqueous compositions and thereby to improve the performance properties of the compositions. In addition, there is a need for a process to disperse inorganic material particles at high concentrations for use in coating and related types of compositions with a minimal amount of dispersing surfactant and with the substantial absence of grit.